1. Field of the Invention
The present invention relates generally to a public address system, and is directed more particularly to a public address system with a device for eliminating howling.
2. Description of the Prior Art
In a speaker or loudspeaker assembly in which one or a plurality of speakers are installed in a hall and a speech received by a microphone is emitted by the speakers, a howling is caused when the microphone is brought near the speakers. To avoid this howling, there has been proposed such a method in which a transmitter generating a constant signal is located near a microphone, while a receiver is located near a speaker, so that when the microphone comes near the speaker, the signal from the transmitter is received by the receiver and then the signal supplied from the microphone to the speaker is attenuated to avoid the howling. In this case, a transmitter and a receiver, each utilizing the electromagnetic induction, are practically used as the above-mentioned transmitter and receiver, respectively. That is, the transmitter is formed such that a current of high frequency, for example, 1 MH.sub.z flows through its transmitting coil to emit an induction magnetic flux while the receiver is so formed that its receiving coil detects the induction magnetic flux from the transmitter.
In this case, the level of the voltage induced across the receiving coil is varied in response to the angle of the induction magnetic flux from the transmitting coil with respect to the receiving coil, and when the induction magnetic flux is perpendicular to the receiving coil, no voltage is induced in the receiving coil. Thus, the operation is not reliable and if the microphone is moved near the speaker, prevention of howling is not assured.